The invention relates to a method for transferring solid or liquid printing ink from an intermediate carrier, such as a transfer cylinder, to a further intermediate carrier or a substrate, such as paper, as well as to a device for performing the method.
In order to be able to transfer liquid printing ink from a cylinder of a printing press onto a further cylinder and a substrate, respectively, the adhesion of the printing ink to the second cylinder or the substrate, a characteristic which is based upon physical interfacial effects, must be greater than the adhesion thereof to the original cylinder. Upon the transfer of liquid ink, however, a cracking or splitting of the liquid film occurs, so that a portion thereof remains on the original cylinder, and it is virtually impossible to achieve anything even remotely close to 100% transfer of liquid printing ink.
The same problem exists in the transfer of solid printing ink which is in a granular state, such as toner powder, for example. To be sure, electrostatic transfer techniques which achieve a transfer efficiency of approximately 95 to 98% maximum have become known heretofore; however, this is only for applications using non-conducting toner. For production presses with an output of many thousands of sheets per hour, this is not sufficient, however, because it would be necessary permanently for the cleaning devices to be replaced and to be cleaned outside the printing press, respectively.
In an article entitled xe2x80x9cOffset Quality Electrophotographyxe2x80x9d in the xe2x80x9cjournal of Imaging Science and Technologyxe2x80x9d, Volume 37, No. 5, September/October 1993, p. 458 (hereinafter referred to as xe2x80x9cOQExe2x80x9d), various xerographic techniques which are suitable for transferring conductive toner even when the humidity is quite high are described. In one of the techniques, the toner is transferred under pressure onto a substrate and is simultaneously fixed. A further technique calls for a thermal or heat transfer in two temperature stages. In addition, combinations of transfer under pressure and thermal transfer are described therein. Such a combination is presented on page 459 of the aforementioned publication. The toner is transferred by pressure from a first cylinder onto an intermediate cylinder and then by thermal transfer onto paper which runs between the intermediate cylinder and a heated impression cylinder. An efficiency of 95% is supposed to be achieved in the case of transfer by pressure, while an efficiency of 100% is attained in the case of thermal transfer.
In the periodical xe2x80x9cThe Seybold Report on Publishing Systemsxe2x80x9d, Volume 24, No. 20, page 20, left-hand column (hereinafter referred to as the xe2x80x9cSeybold Reportxe2x80x9d), a transfer system is described wherein an image is transferred to paper through the intermediary of two belts. One of the belts accepts the toner in a distribution corresponding to the printed image. The image is then transferred to the other belt, which is heated. The latter belt is not hot enough to melt the toner, but the heat thereof is sufficient to cause the toner particles to adhere to one another. The heated belt then transfers the image onto the paper, which is preheated, the image being fixed by means of a hot pressure roller. Consequently, no subsequent melting or fixing of the toner is required.
In the first step, the transfer from a first belt to a second belt, it is not possible at all to achieve a transfer efficiency of 100%. The first belt is, in fact, teflon-coated, and basically exerts at least small adhesion forces on the toner, so that, in the first step at least, a transfer efficiency of less than 100% is to be assumed, as is similar to the situation with the technique described in the aforementioned xe2x80x9cOffset Quality Electrophotographyxe2x80x9d article.
In both of the aforedescribed techniques, the toner is thus not transferred in its entirety from the cylinder and from the first belt, respectively. Particularly with regard to recently developed printing techniques, e.g., the printing technique described in the xe2x80x9cSeybold Reportxe2x80x9d, it is necessary, however, for the remaining printing ink to be entirely removed before new printing ink is applied to the first belt and to the first cylinder, respectively, in order to attain a perfect print. This may be very difficult and costly, particularly if the printing ink is to be reused.
It is accordingly an object of the invention to provide a method and a device for transferring printing ink wherein transfer techniques are used which virtually always have a transfer efficiency of 100%, not only when transferring ink onto paper, but also when transferring ink onto an intermediate carrier.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method of transferring printing ink from an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, wherein the printing ink adheres in a granular state to the intermediate carrier, which comprises melting the printing ink at a side thereof facing away from the intermediate carrier before transferring the printing ink to the printing-ink receiver.
In accordance with another aspect of the invention, there is provided a method of transferring printing ink from an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, wherein the printing ink adheres in an at least partially liquid state to the intermediate carrier, which comprises reducing the adhesion of the printing ink to the intermediate carrier with a separating agent at one time at least before and during transfer to the printing-ink receiver.
In accordance with a further mode, wherein the intermediate carrier has an outer elastic layer containing the separating agent, the method according to the invention includes pressing the intermediate layer and the printing-ink receiver against one another so as to drive the separating agent to the surface of the elastic layer.
In accordance with an added aspect of the invention, there is provided a device for transferring printing ink from an inking unit via an intermediate carrier to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, comprising a heat source disposed opposite a surface of the intermediate carrier, the surface extending between the inking unit and the printing-ink receiver.
In accordance with an additional aspect of the invention, there is provided a device for transferring printing ink from an inking unit, comprising an intermediate carrier for receiving printing ink from the inking unit and for transferring the printing ink to a printing-ink receiver selected from the group consisting of a further intermediate carrier and a substrate, the intermediate carrier having at least one of the properties consisting of being permeable to a separating agent and having a storage capacity for a separating agent.
In accordance with yet another aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit and having a surface to which printing ink is transferrable from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate and having a surface extending between the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, comprising a first heat source disposed opposite the surface of the first intermediate carrier, and a second heat source disposed opposite the surface of the second intermediate carrier.
In accordance with yet a further aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit and having a surface to which printing ink is transferrable from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate and having a surface extending between the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, comprising a first heat source disposed opposite the surface of the first intermediate carrier, and the second intermediate carrier being provided with at least one of a second heat source disposed opposite the surface of the second intermediate carrier and a capability for being permeable to a separating agent at a side thereof facing the printing ink.
In accordance with yet another aspect of the invention, there is provided a device for transferring printing ink from an inking unit of a printing press to a substrate being transported through the printing press, comprising a first intermediate carrier adjoining the inking unit for receiving thereon printing ink transferred from the printing unit, and a second intermediate carrier disposed so as to be in contact with the first intermediate carrier and the substrate for transferring printing ink from the first intermediate carrier to the substrate, both the first intermediate carrier and the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink.
In accordance with another feature of the invention, the device in a printing press having a plurality of inking units includes a plurality of first intermediate carriers and a second intermediate carrier, each of the first intermediate carriers, respectively, adjoining one of the inking units and the second intermediate carrier, the second intermediate carrier being disposed so as to contact all of the first intermediate carriers and a substrate being transported through the printing press, and a heat source disposed opposite a surface of the second intermediate carrier, the surface extending between at least one of the first intermediate carriers and the substrate.
In accordance with a further feature of the invention, the device in a printing press having a plurality of inking units includes a plurality of first intermediate carriers and a second intermediate carrier, each of the first intermediate carriers, respectively, adjoining one of the inking units and the second intermediate carrier, the second intermediate carrier being disposed so as to contact all of the first intermediate carriers and a substrate being transported through the printing press, the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink.
In accordance with an added feature of the invention, the device in a printing press having a plurality of inking units includes a first intermediate carrier and a second intermediate carrier, each of the plurality of inking units adjoining the first intermediate carrier and the second intermediate carrier, the second intermediate carrier being disposed so as to contact the first intermediate carrier and a substrate being transported through the printing press, and a heat source disposed opposite a surface of the second intermediate carrier, the surface extending between the first intermediate carrier and the substrate.
In accordance with an additional feature of the invention, the device in a printing press having a plurality of inking units includes a first intermediate carrier and a second intermediate carrier, each of the plurality of inking units adjoining the first intermediate carrier and the second intermediate carrier, the second intermediate carrier being disposed so as to contact the first intermediate carrier and a substrate being transported through the printing press, the second intermediate carrier being permeable to a separating agent at a side thereof facing the printing ink.
In accordance with yet another feature of the invention, the intermediate carrier contacting the substrate has an outer elastic layer containing the separating agent therein.
In accordance with yet a further feature of the invention, the separating agent is an ink-repelling liquid.
In accordance with yet an added feature of the invention, the ink-repelling liquid is silicone oil.
In accordance with yet an additional feature of the invention, the intermediate carrier is one of a rotating transfer cylinder and a belt revolving around a cylinder.
In accordance with still another feature of the invention, the device includes an impression cylinder journalled opposite a side of the substrate whereon a surface of the one of the rotating transfer cylinder and the belt revolving around a cylinder rolls.
In accordance with still a further feature of the invention, the heat source is formed so as to concentrate radiation on one of the intermediate carrier and the substrate.
In accordance with still an added feature of the invention, the heat source is one of a laser, an array of lasers and an array of laser diodes.
In accordance with a concomitant feature of the invention, the device includes a control device for generating control signals connected to the one of the laser, the array of lasers and the array of laser diodes, the control signals corresponding to a distribution of the printing ink on the printing-ink receiver.
It is thus a more specific objective of the invention, when, in the aforementioned method, the printing ink adheres in a granular state to an intermediate carrier, to effect an initial melting of the printing ink at a side thereof facing away from the intermediate carrier, before the printing ink is transferred to another intermediate carrier or to the substrate.
It is a further specific objective of the invention, when, in the aforementioned method, the printing ink adheres in an at least partially liquid state to the intermediate carrier, to reduce the adhesion of the printing ink to the intermediate carrier by applying a separating agent before and/or during the ink transfer to the other intermediate carrier or to the substrate.
In an embodiment of the invention, the intermediate carrier has an outer elastic layer, such as a rubber layer, the separating agent being contained in the layer and the separating agent being driven to the surface of the rubber layer when the intermediate carrier and either the further intermediate carrier or the substrate, as the case may be, are pressed against one another. In a further embodiment relating to the use of a separating agent, the intermediate carrier has a hard outer layer, such as a porous layer formed of sintered material or ceramic, the outer layer being capable of effectively storing a separating agent therein. The loss of separating agent occurring during operation can be compensated for by a corresponding feeding device, or the separating agent is added to the ink, so that an equilibrium in the supply of separating agent is attained during operation.
An intermediate carrier is understood to mean a device having a surface whereon a printed image in the form of an ink distribution is created, further transported and subsequently destroyed or removed, i.e., for example, a rotating transfer cylinder or a belt revolving around a cylinder, the ink being transferred either onto a further intermediate carrier or onto a substrate.
A transfer efficiency of virtually 100% is achieved both in the case of the transfer of solid ink according to the invention, i.e. transfer by a melting start-up, the printing ink being predominantly solid, and also in the case of the transfer of liquid inks assisted by a separating agent.
In the case of the start-up melting of a granular printing ink from outside, assurance is provided, firstly, that the ink particles adhere to one another. Secondly, the adhesion of the printing ink to the intermediate carrier is not increased by the start-up melting, because the start-up melting takes place only on the outer surface of the ink film, while, on the side facing the intermediate carrier, the printing ink continues to adhere only at individual points to the intermediate carrier. Therefore, coherent ink islands on the intermediate carrier can easily and completely be removed therefrom. Thirdly, the start-up or initial melting of the printing ink from outside results in a stronger adhesion thereof to the following intermediate carrier or substrate, thereby additionally ensuring the complete transfer of the printing ink.
Otherwise than in the process known from the xe2x80x9cSeybold Reportxe2x80x9d, according to the invention of the instant application, the solid printing ink does not undergo start-up or initial melting from the inside, but rather, on the outside. Whereas the conventional technology makes the stripping or removal removal of the printing ink from the intermediate carrier more difficult, the stripping or removal forces required in accordance with the invention continue to remain small and, moreover, the adhesion on the target carrier is improved. Consequently, in the method for transferring solid ink, according to the invention, the transfer method is in a plurality of respects more reliable than in the conventional process. With little design effort and with little expenditure of energy, a transfer efficiency of 100% is attained, the contact-pressure forces required for transfer onto a substrate such as paper being small, so that the paper is treated more gently.
This applies not only when the printing ink is transferred directly onto a substrate such as paper, which assists transfer due to the strong capillary action thereof, but also in the case of a multistage process wherein the printing ink is transferred onto the substrate via a further intermediate carrier. The complete transfer of a latent image, developed on a first intermediate carrier, onto a second intermediate carrier and from there onto a substrate imposes contradicting requirements on the two processes. First of all, the affinity of the printing ink to the second intermediate carrier must be greater than the affinity thereof to the first intermediate carrier; secondly, the affinity of the printing ink to the substrate must be greater than the affinity thereof to the second intermediate carrier. Therefore, the second intermediate carrier must initially accept the printing ink before then releasing it again. Because, in accordance with the invention, the stripping or removal of the printing ink from the first intermediate carrier after the development thereof is facilitated, a lesser affinity is required on the second intermediate carrier than if the stripping or removal were not assisted, in order for the ink to be transferred completely onto the second intermediate carrier. The low adhesion forces on the second intermediate carrier make it easier for the printing ink subsequently to be transferred completely onto the substrate.
If the start-up melting method according to the invention is likewise used for transfer onto the substrate, then the method of transfer and final fixing require overall considerably less heat energy than in conventional techniques, wherein the printing ink is melted by heating the intermediate carrier or by preheating the paper. The paper does not dry out during printing and is treated more gently.
If the printing ink is on the intermediate carrier in an at least partially liquid state or has been brought to such a state, then, in accordance with the invention, a separating agent is used in order to ensure 100% transfer. The printing ink may be either an ink which is liquid at normal temperature, or a meltable ink which is solid at normal temperature and is kept at a temperature above the melting temperature. In the latter case, for example, the intermediate carrier is provided with a heated outer layer of high thermal conductivity and low thermal capacity and with an insulating layer lying therebelow in order to keep the heat losses low.
The use of a separating agent is particularly advantageous if the intermediate carrier is in the form of a rubber-covered transfer cylinder which transfers the liquid printing ink onto a substrate. An elastic layer of the transfer cylinder formed, for example, of rubber or a similar material and being able huggingly to adapt to an uneven substrate surface in order to ensure uniform ink transfer without having to exert excessive pressure, serves simultaneously as the carrier for the separating agent which, in the preferred embodiment, is silicone oil. The capacity of the elastic layer to absorb the separating agent may be based on diffusion and/or on the penetration of the separating agent into micropores of the elastic layer. When the elastic layer is pressed against the substrate, the silicone oil is driven out, so that the printing ink is repelled from the surface of the transfer cylinder. At the same time, the printing ink is driven into the substrate surface, so that the complete transfer of the ink is achieved in an especially simple manner.
In the case of more modern inking units, particularly in the case of digital inking units, it is necessary, for technical reasons, that the printing ink be applied initially to a first intermediate carrier which has a hard surface. If the printing ink is liquid, in this case, provision may be made for the first intermediate carrier to be formed with micropores through which a separating agent may be pressed, before and/or during the transfer of the printing ink, onto a second intermediate carrier, the separating gas being, in this case, not restricted to a liquid only, but also possibly being a gas. If the printing ink, conversely, is originally solid, then the solid-ink transfer according to the invention is performed on the first intermediate carrier, while either the solid-ink transfer or the liquid-ink transfer according to the invention is performed on the second intermediate carrier.
A printing press with one or more printing units according to the invention includes a transport device, such as a conventional conveyor with chains and grippers or a transport belt, the transport device conveying substrates consecutively through the in-line printing units, the substrates being pressed against the corresponding intermediate carriers by means of impression cylinders.
Furthermore, due to the high transfer efficiency, simplified constructions for multicolor printing presses are possible.
The heat sources used in the various embodiments for start-up or initial melting or for fixing may be, for example, infrared radiators which concentrate the radiation onto the intermediate carrier or substrate. The location on the intermediate carrier at which the radiation is concentrated should be as close as possible to the location at which the printing ink is transferred onto the further intermediate carrier or substrate, so that, on the travel path of the printing ink to the transfer location, as little heat as possible flows onto the intermediate carrier and the printing ink need not be heated to a considerably higher temperature than is required for transfer, respectively.
Best suited for the concentration of the radiation are lasers, the radiation of which is converted at the point of incidence into heat. Through a suitable choice of the radiation wavelength, it is possible to ensure that the radiation be absorbed with a higher efficiency by the printing ink and with a lower efficiency by the intermediate carrier or substrate, respectively, so that the intermediate carrier and the substrate are heated as little as possible. Even more selective heating is possible with the aid of an array of lasers or of laser diodes, which are controlled in conformance with the transferred printing image, in order to heat only those locations which bear the printing ink. The information required for generating such a heating pattern is known from the control of the imaging heads. Given a corresponding resolution of the laser-diode array, it is possible to implement heat transfer to pixel accuracy and, with the aid of the gray-value information, it is possible to take into account the respective ink-film thickness. Consequently, the supply of heat can be measured so that, upon the transfer of the printing ink to the paper, the printing ink has the same temperature overall, irrespective of other parameters, thereby assuring a reliable transfer of the ink.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as a method and device for transferring printing ink, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: